Automation Technologies

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Part X Manufacturing
Systems
38
AUTOMATION
TECHNOLOGIES FOR
MANUFACTURING
SYSTEMS
Chapter Contents
38.1 Automation Fundamentals
38.1.1 Three Components of an Automated
System
38.1.2 Types of Automation
38.2 Hardware Components for Automation
38.2.1 Sensors
38.2.2 Actuators
38.2.3 Interface Devices
38.2.4 Process Controllers
38.3 Numerical Control
38.3.1 The Technology of Numerical Control
38.3.2 Analysis of NC Positioning Systems
38.3.3 NC Part Programming
38.3.4 Applications of Numerical Control
38.4 Industrial Robotics
38.4.1 Robot Anatomy
38.4.2 Control Systems and Robot
Programming
38.4.3 Applications of Industrial Robots
In this part of the book, we consider the manufacturing
systems that are commonly associated with the production
and assembly processes discussed in preceding chapters. A
manufacturing system can be defined as a collection of inte-
grated equipment and human resources that performs one or
more processing and/or assembly operations on a starting
work material, part, or set of parts. The integrated equipment
consists of production machines, material handling and posi-
tioning devices, and computer systems. Human resources are
required either full-time or part-time to keep the equipment
operating. The position of the manufacturing systems in the
larger production system is shown in Figure 38.1. As the
diagram indicates, the manufacturing systems are located in
the factory. They accomplish the value-added work on the part
or product.
Manufacturing systems include both automated and
manually operated systems. The distinction between the two
categories is not always clear, because many manufacturing
systems consist of both automated and manual work ele-
ments (e.g., a machine tool that operates on a semiautomatic
processing cycle but which must be loaded and unloaded
each cycle by a human worker). Our coverage includes both
categories and is organized into two chapters: Chapter 38 on
automation technologies and Chapter 39 on integrated
manufacturing systems. Chapter 38 provides anintroductory
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treatment of automation technology and the components that make up an automated
system. We also discuss two important automation technologies used in manufacturing:
numerical control andindustrial robotics. InChapter 39, weexaminehowtheseautomation
technologies are integrated into more sophisticated manufacturing systems. Topics include
production lines, cellular manufacturing, flexible manufacturing systems, and computer
integrated manufacturing. A more detailed discussion of the topics in these two chapters
can be found in [5].
38.1 AUTOMATION FUNDAMENTALS
Automation can be defined as the technology by which a process or procedure is
performed without human assistance. Humans may be present as observers or even
participants, but the process itself operates under its own self-direction. Automation
is implemented by means of a control system that executes a program of instructions.
To automate a process, power is required to operate the control system and to drive
the process itself.
38.1.1 THREE COMPONENTS OF AN AUTOMATED SYSTEM
As indicated above, an automated system consists of three basic components: (1) power,
(2) a program of instructions, and (3) a control system to carry out the instructions. The
relationship among these components is shown in Figure 38.2.
The form of power used in most automated systems is electrical. The advantages of
electrical power include (1) it is widely available, (2) it can be readily converted to other
forms of power such as mechanical, thermal, or hydraulic, (3) it can be used at very low
power levels for functions such as signal processing, communication, data storage, and
data processing, and (4) it can be stored in long-life batteries [5].
FIGURE 38.1 The
position of the manufac-
turing systems in the
larger production system.
Manufacturing processes and assembly operations
Facilities
Manufacturing
support
Quality control
system
Manufacturing
systems
Manufacturing
support systems
Production system
Finished
products
Engineering
materials
FIGURE 38.2 Elements
of an automated system:
(1) power, (2) program
of instructions, and (3)
control system.
(1) Power
(2) Program of
instructions
(3) Control
system
Process
Process output
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In a manufacturing process, power is required to accomplish the activities associated
with the particular process. Examples of these activities include (1) melting a metal in a
casting operation, (2) driving the motions of a cutting tool relative to a workpiece in a
machining operation, and (3) pressing and sintering parts in a powder metallurgy process.
Power is also used to accomplish any material handling activities needed in the process,
such as loading and unloading parts, if these activities are not performed manually. Finally,
power is used to operate the control system.
The activities in an automated process are determined by a program of instructions. In
the simplest automated processes, the only instruction may be to maintain a certain
controlled variable at a specified level, such as regulating the temperature in a heat treatment
furnace. In more complex processes, a sequence of activities is required during the work
cycle, and the order and details of each activity are defined by the program of instructions.
Each activity involves changes in one or more process parameters, such as changing the x-
coordinate position of a machine tool worktable, opening or closing a valve in a fluid flow
system, or turning a motor on or off. Process parameters are inputs to the process. They may
be continuous (continuously variable over a given range, such as the x-position of a
worktable) or discrete (On or Off). Their values affect the outputs of the process, which
are called process variables. Like process parameters, process variables can be continuous
or discrete. Examples include the actual position of the machine worktable, the rotational
speed of a motor shaft, or whether a warning light is on or off. The program of instructions
specifies the changes in process parameters and when they should occur during the work
cycle, and these changes determine the resulting values of the process variables. For example,
in computer numerical control, the program of instructions is called a part program. The
numerical control (NC) part program specifies the individual sequence of steps required to
machine a given part, including worktable and cutter positions, cutting speeds, feeds, and
other details of the operation.
In some automated processes, the work cycle programmust contain instructions for
making decisions or reacting to unexpected events during the work cycle. Examples of
situations requiring this kind of capability include (1) variations in raw materials that
require adjusting certain process parameters to compensate, (2) interactions and com-
munications with human such as responding to requests for system status information,
(3) safety monitoring requirements, and (4) equipment malfunctions.
The program of instructions is executed by a control system, the third basic
component of an automated system. Two types of control system can be distinguished:
closed loop and open loop. Aclosed loop system, also known as a feedback control system,
is one in which the process variable of interest (output of the process) is compared with the
corresponding process parameter (input to the process), and any difference between them
is used to drive the output value into agreement with the input. Figure 38.3(a) shows the six
elements of a closed loop system: (1) input parameter, (2) process, (3) output variable, (4)
feedback sensor, (5) controller, and (6) actuator. The input parameter represents the
desired value of the output variable. The process is the operation or activity being
controlled; more specifically, the output variable is being controlled by the system. A
sensor is used to measure the output variable and feed back its value to the controller,
which compares output with input and makes the required adjustment to reduce any
difference. The adjustment is made by means of one or more actuators, which are
hardware devices that physically accomplish the control actions.
The other type of control systemis an open loop system, presented in Figure 38.3(b).
As shown in the diagram, an open loop system executes the program of instructions
without a feedback loop. No measurement of the output variable is made, so there is no
comparison between output and input in an open loop system. In effect, the controller
relies on the expectation that the actuator will have the intended effect on the output
variable. Thus, there is always a risk in an open loop system that the actuator will not
function properly or that its actuation will not have the expected effect on the output. On
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the other hand, the advantage of an open loop system is that its cost is less than a
comparable closed loop system.
38.1.2 TYPES OF AUTOMATION
Automated systems used in manufacturing can be classified into three basic types:
(1) fixed automation, (2) programmable automation, and (3) flexible automation.
Fixed Automation In fixed automation, the processing or assembly steps and their
sequence are fixed by the equipment configuration. The program of instructions is deter-
mined by the equipment design and cannot be easily changed. Each step in the sequence
usually involves a simple action, such as feeding a rotating spindle along a linear trajectory.
Although the work cycle consists of simple operations, integrating and coordinating the
actions can result in the need for a rather sophisticated control system, and computer control
is often required.
Typical features of fixed automation include (1) high initial investment for specialized
equipment, (2) high production rates, and (3) little or no flexibility to accommodate product
variety. Automated systems with these features can be justified for parts and products that
are produced in very large quantities. The high investment cost can be spread over many
units, thus making the cost per unit relatively low compared to alternative production
methods. The automated production lines discussed in the following chapter are examples
of fixed automation.
Programmable Automation As its name suggests, the equipment in programmable
automation is designed with the capability to change the program of instructions to allow
production of different parts or products. New programs can be prepared for newparts, and
the equipment can read each program and execute the encoded instructions. Thus the
features that characterize programmable automation are (1) high investment in general
purpose equipment that can be reprogrammed, (2) lower production rates than fixed
automation, (3) ability to cope with product variety by reprogramming the equipment, and
(4) suitability for batch production of various part or product styles. Examples of program-
mable automation include computer numerical control and industrial robotics, discussed in
Sections 38.3 and 38.4, respectively.
Flexible Automation Suitability for batch production is mentioned as one of the
features of programmable automation. As discussed in Chapter 1, the disadvantage
FIGURE 38.3 Two basic
types of control systems:
(a) closed loop and
(b) open loop.
Controller
(1)
Input
parameter
(3)
Output
variable
Input
parameter
(5) (6)
(4)
(a)
(b)
(2)
Actuator
Feedback
sensor
Process
Controller Actuator Process
Output
variable
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of batch production is that lost production time occurs between batches due to
equipment and/or tooling changeovers that are required to accommodate the next
batch. Thus, programmable automation usually suffers from this disadvantage.
Flexible automation is an extension of programmable automation in which there
is virtually no lost production time for setup changes and/or reprogramming. Any
required changes in the program of instructions and/or setup can be accomplished
quickly; that is, within the time needed to move the next work unit into position at the
machine. A flexible system is therefore capable of producing a mixture of different
parts or products one right after the other instead of in batches. Features usually
associated with flexible automation include (1) high investment cost for custom-
engineered equipment, (2) medium production rates, and (3) continuous production
of different part or product styles.
Using some terminology developed in Chapter 1, we might say that fixed automa-
tion is applicable in situations of hard product variety, programmable automation is
applicable to medium product variety, and flexible automation can be used for soft
product variety.
38.2 HARDWARE COMPONENTS FOR AUTOMATION
Automation and process control are implemented using various hardware devices that
interact with the production operation and associated processing equipment. Sensors are
required to measure the process variables. Actuators are used to drive the process
parameters. And various additional devices are needed to interface the sensors and
actuators with the process controller, which is usually a digital computer.
38.2.1 SENSORS
A sensor is a device that converts a physical stimulus or variable of interest (e.g.,
temperature, force, pressure, or other characteristic of the process) into a more convenient
physical form (e.g., electrical voltage) for the purpose of measuring the variable. The
conversion allows the variable to be interpreted as a quantitative value.
Sensors of various types are available to collect data for feedback control in
manufacturing automation. They are often classified according to type of stimulus;
thus, we have mechanical, electrical, thermal, radiation, magnetic, and chemical variables.
Within each category, there are multiple variables that can be measured. Within the
mechanical category, the physical variables include position, velocity, force, torque, and
many others. Electrical variables include voltage, current, and resistance. And so on for
the other major categories.
In addition to type of stimulus, sensors are also classified as analog or discrete. An
analog sensor measures a continuous analog variable and converts it into a continuous
signal such as electrical voltage. Thermocouples, strain gages, and ammeters are exam-
ples of analog sensors. A discrete sensor produces a signal that can have only a limited
number of values. Within this category, we have binary sensors and digital sensors. A
binary sensor can take on only two possible values, such as Off and On, or 0 and 1. Limit
switches operate this way. A digital sensor produces a digital output signal, either in the
form of parallel status bits, such as a photoelectric sensor array) or a series of pulses that
can be counted, such as an optical encoder. Digital sensors have an advantage that they can
be readily interfaced to a digital computer, whereas the signals from analog sensors must
be converted to digital in order to be read by the computer.
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For a given sensor, there is a relationship between the value of the physical stimulus
and the value of the signal produced by the sensor. This input/output relationship is called
the sensor’s transfer function, which can be expressed as:
S ¼ f s ð Þ ð38:1Þ
where S ¼ the output signal of the sensor (typically voltage), s ¼ the stimulus or input,
and f(s) is the functional relationship between them. The ideal form for an analog sensor
is a proportional relationship:
S ¼ C þ ms ð38:2Þ
where C ¼ the value of the sensor output when the stimulus value is zero, and m ¼ the
constant of proportionality between s and S. The constant m indicates how much the
output S is affected by the input s. This is referred to as the sensitivity of the measuring
device. For example, a standard Chromel/Alumel thermocouple produces 40.6 micro-
volts per

C change in temperature.
A binary sensor (e.g., limit switch, photoelectric switch) exhibits a binary relation-
ship between stimulus and sensor output:
S ¼ 1 if s > 0 and S ¼ 0 if s 0 ð38:3Þ
Before a measuring device can be used, it must be calibrated, which basically means
determining the transfer function of the sensor; specifically, how is the value of the
stimulus s determined from the value of the output signal S? Ease of calibration is one
criterion by which a measuring device can be selected. Other criteria include accuracy,
precision, operating range, speed of response, reliability and cost.
38.2.2 ACTUATORS
In automated systems, an actuator is a device that converts a control signal into a physical
action, which usually refers to a change in a process input parameter. The action is
typically mechanical, such as a change in position of a worktable or rotational speed of a
motor. The control signal is generally a low level signal, and an amplifier may be required
to increase the power of the signal to drive the actuator.
Actuators can be classified according to type of amplifier as (1) electrical, (2) hy-
draulic, or (3) pneumatic. Electrical actuators include AC and DC electric motors, stepper
motors, and solenoids. The operations of two types of electric motors (servomotors and
stepper motors) are described in Section 38.3.2, which deals with the analysis of positioning
systems. Hydraulic actuators utilize hydraulic fluid to amplify the control signal and are
often specified when large forces are required in the application. Pneumatic actuators are
driven by compressed air, which is commonly used in factories. All three actuator types are
available as linear or rotational devices. This designation distinguishes whether the output
action is a linear motion or a rotational motion. Electric motors and stepper motors are
more common as rotational actuators, whereas most hydraulic and pneumatic actuators
provide a linear output.
38.2.3 INTERFACE DEVICES
Interface devices allow the process to be connected to the computer controller and vice
versa. Sensor signals from the manufacturing process are fed into the computer, and
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command signals are sent to actuators that operate the process. In this section, we discuss
the hardware devices that enable this communication between the process and the
controller. The devices include analog-to-digital converters, digital-to-analog converters,
contact input/output interfaces, and pulse counters and generators.
Continuous analog signals from sensors attached to the process must be trans-
formed into digital values that can be used by the control computer, a function that is
accomplished by an analog-to-digital converter (ADC). As illustrated in Figure 38.4,
an ADC (1) samples the continuous signal at periodic intervals, (2) converts the
sampled data into one of a finite number of defined amplitude levels, and (3) encodes
each amplitude level into a sequence of binary digits that can be interpreted by the
control computer. Important characteristics of an analog-to-digital converter include
sampling rate and resolution. Sampling rate is the frequency with which the continuous
signal is sampled. A faster sampling rate means that the actual form of the continuous
signal can be more closely approximated. Resolution refers to the precision with which
the analog value can be converted into binary code. This depends on the number of bits
used in the encoding procedure, the more bits, the higher the resolution. Un-
fortunately, using more bits requires more time to make the conversion, which can
impose a practical limit on the sampling rate.
A digital-to-analog converter (DAC) accomplishes the reverse process of the
ADC. It converts the digital output of the control computer into a quasi-continuous
signal capable of driving an analog actuator or other analog device. The DACperforms its
function in two steps: (1) decoding, in which the sequence of digital output values is
transformed into a corresponding series of analog values at discrete time intervals, and
(2) data holding, in which each analog value is changed into a continuous signal during
the duration of the time interval. In the simplest case, the continuous signal consists of a
series of step functions, as in Figure 38.5, which are used to drive the analog actuator.
FIGURE 38.4
An analog-to-digital
converter works by
converting a continuous
analog signal into a series
of discrete sampled data.
Discrete
sampled signal
Continuous analog signal
Variable
Time
FIGURE 38.5
An analog-to-digital
converter works by
converting a continuous
analog signal into a series
of discrete sampled data.
Time
Series of discrete
step functions
Ideal output envelope
Parameter
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Many automated systems operate by turning on and off motors, switches, and other
devices to respond to conditions and as a function of time. These control devices use
binary variables. They can have either of two possible values, 1 or 0, interpreted as On or
Off, object present or not present, high or low voltage level, and so on. Binary sensors
commonly used in process control systems include limit switches and photocells.
Common binary actuators solenoids, valves, clutches, lights, control relays, and certain
motors.
Contact input/output interfaces are components used to communicate binary data
back and forth between the process and the control computer. A contact input interface
is a device that reads binary data into the computer from an external source. It consists of
a series of binary electrical contacts that indicate the status of a binary device such as a
limit switch attached to the process. The status of each contact is periodically scanned by
the computer to update values used by the control program. Acontact output interface is
a device used to communicate on/off signals from the computer to external binary
components such as solenoids, alarms, and indicator lights. It can also be used to turn on
and off constant speed motors.
As mentioned earlier, discrete data sometimes exist in the formof a series of pulses.
For example, an optical encoder (discussed in Section 38.3.2) emits its measurement of
position and velocity as a series of pulses. Apulse counter is a device that converts a series
of pulses from an external source into a digital value, which is entered into the control
computer. In addition to reading the output of an optical encoder, applications of pulse
counters include counting the number of parts flowing along a conveyor past a photo-
electric sensor. The opposite of a pulse counter is a pulse generator, a device that
produces a series of electrical pulses based on digital values generated by a control
computer. Both the number and frequency of the pulses are controlled. An important
pulse generator application is to drive stepper motors, which respond to each step by
rotating through a small incremental angle, called a step angle.
38.2.4 PROCESS CONTROLLERS
Most process control systems use some type of digital computer as the controller.
Whether control involves continuous or discrete parameters and variables, or a
combination of continuous and discrete, a digital computer can be connected to
the process to communicate and interact with it using the interface devices discussed
in Section 38.2.3. Requirements generally associated with real-time computer control
include the following:
å The capability of the computer to respond to incoming signals from the process and if
necessary, to interrupt execution of a current program to service the incoming signal.
å The capability to transmit commands to the process that are implemented by means
of actuators connected to the process. These commands may be the response to
incoming signals from the process.
å The capability to execute certain actions at specific points in time during process
operation.
å The capability to communicate and interact with other computers that may be
connected to the process. The term distributed process control is used to describe a
control system in which multiple microcomputers are used to share the process
control workload.
å The capability to accept input fromoperating personnel for purposes such as entering
new programs or data, editing existing programs, and stopping the process in an
emergency.
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A widely used process controller that satisfies these requirements is a program-
mable logic controller. A programmable logic controller (PLC) is a microcomputer-
based controller that uses stored instructions in programmable memory to implement
logic, sequencing, timing, counting, and arithmetic control functions, through digital or
analog input/output modules, for controlling various machines and processes. The major
components of a PLC, shown in Figure 38.6, are (1) input and output modules, which
connect the PLC to the industrial equipment to be controlled; (2) processor—the central
processing unit (CPU), which executes the logic and sequencing functions to control the
process by operating on the input signals and determining the proper output signals
specified by the control program; (3) PLC memory, which is connected to the processor
and contains the logic and sequencing instructions; (4) power supply—115 V AC is
typically used to drive the PLC. In addition, (5) a programming device (usually
detachable) is used to enter the program into the PLC.
Programming involves entry of the control instructions to the PLC using the
programming device. The most common control instructions include logical operations,
sequencing, counting, and timing. Many control applications require additional instruc-
tions for analog control, data processing, and computations. Avariety of PLC program-
ming languages have been developed, ranging from ladder logic diagrams to structured
text. A discussion of these languages is beyond the scope of this text, and the reader is
referred to our references.
Advantages associated with programmable logic controllers include (1) program-
ming a PLC is easier than wiring a relay control panel; (2) PLCs can be reprogrammed,
whereas conventional hard-wired controls must be rewired and are often scrapped
instead because of the difficulty in rewiring; (3) a PLC can be interfaced with the plant
computer system more readily than conventional controls; (4) PLCs require less floor
space than relay controls, and (5) PLCs offer greater reliability and easier maintenance.
38.3 COMPUTER NUMERICAL CONTROL
Numerical control (NC) is a form of programmable automation in which the mechanical
actions of a piece of equipment are controlled by a program containing coded alphanu-
meric data. The data represent relative positions between a workhead and a workpart.
The workhead is a tool or other processing element, and the workpart is the object being
processed. The operating principle of NC is to control the motion of the workhead
FIGURE 38.6 Major
components of a
programmable logic
controller.
External power source
Outputs to
process
Inputs from
process
(3)
(2)
(4)
(5)
(1)
Programming
device
Power
supply
Memory
Input
module
Output
module
Processor
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relative to the workpart and to control the sequence in which the motions are carried out.
The first application of numerical control was in machining (Historical Note 38.1), and
this is still an important application area. NC machine tools are shown in Figures 22.26
and 22.27. Our video clip on computer numerical control shows the various types of CNC
machines and operations.
VIDEO CLIP
Computer Numerical Control. The clip contains two segments: (1) computer numerical
controls and (2) CNC principles.
38.3.1 THE TECHNOLOGY OF NUMERICAL CONTROL
In this section we define the components of a numerical control system, and then proceed
to describe the coordinate axis system and motion controls.
Components of an NC System A numerical control system consists of three basic
components: (1) part program, (2) machine control unit, and (3) processing equipment.
The part program (the term commonly used in machine tool technology) is the detailed
set of commands to be followed by the processing equipment. It is the program of
instructions in the NC control system. Each command specifies a position or motion that
is to be accomplished by the work head relative to the workpart. A position is defined by
its x-y-z coordinates. In machine tool applications, additional details in the NC program
include spindle rotation speed, spindle direction, feed rate, tool change instructions, and
Historical Note 38.1 Numerical control [3], [5]
The initial development work on numerical control is
credited to John Parsons and Frank Stulen at the Parsons
Corporation in Michigan in the late 1940s. Parsons was a
machining contractor for the U.S. Air Force and had
devised a means of using numerical coordinate data to
move the worktable of a milling machine for producing
complex parts for aircraft. On the basis of Parson’s work,
the Air Force awarded a contract to the company in 1949
to study the feasibility of the new control concept for
machine tools. The project was subcontracted to the
Massachusetts Institute of Technology to develop a
prototype machine tool that utilized the new numerical
data principle. The M.I.T. study confirmed that the concept
was feasible and proceeded to adapt a three-axis vertical
milling machine using combined analog-digital controls.
The name numerical control (NC) was given to the system
by which the machine tool motions were accomplished.
The prototype machine was demonstrated in 1952.
The accuracy and repeatability of the NC system
was far better than the manual machining methods
then available. The potential for reducing
nonproductive time in the machining cycle was also
apparent. In 1956, the Air Force sponsored the
development of NC machine tools at several different
companies. These machines were placed in operation
at various aircraft plants between 1958 and 1960. The
advantages of NC soon became clear, and aerospace
companies began placing orders for new NC
machines.
The importance of part programming was clear from
the start. The Air Force continued to encourage the
development and application of NC by sponsoring
research at M.I.T. for a part programming language to
control NC machines. This research resulted in the
development of APT in 1958 (APT stands for
Automatically Programmed Tooling). APT is a part
programming language by which a user could write the
machining instructions in simple English-like statements,
and the statements were coded to be interpreted by the
NC system.
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other commands related to the operation. The part program is prepared by a part
programmer, a person who is familiar with the details of the programming language and
also understands the technology of the processing equipment.
The machine control unit (MCU) in modern NC technology is a microcomputer
that stores and executes the program by converting each command into actions by the
processing equipment, one command at a time. The MCU consists of both hardware and
software. The hardware includes the microcomputer, components to interface with the
processing equipment, and certain feedback control elements. The software in the MCU
includes control system software, calculation algorithms, and translation software to
convert the NC part program into a usable format for the MCU. The MCU also permits
the part program to be edited in case the program contains errors, or changes in cutting
conditions are required. Because the MCU is a computer, the term computer numerical
control (CNC) is often used to distinguish this type of NC from its technological
predecessors that were based entirely on hard-wired electronics.
The processing equipment accomplishes the sequence of processing steps to
transform the starting workpart into a completed part. It operates under the control
of the MCU according to the instructions in the part program. We survey the variety of
applications and processing equipment in Section 38.3.4.
Coordinate System and Motion Control in NC A standard coordinate axis system is
used to specify positions in numerical control. The system consists of the three linear axes
(x, y, z) of the Cartesian coordinate system, plus three rotational axes (a, b, c), as shown in
Figure 38.7(a). The rotational axes are used to rotate the workpart to present different
surfaces for machining, or to orient the tool or workhead at some angle relative to the
part. Most NC systems do not require all six axes. The simplest NC systems (e.g., plotters,
pressworking machines for flat sheet-metal stock, and component insertion machines)
are positioning systems whose locations can be defined in an x-y plane. Programming of
these machines involves specifying a sequence of x-y coordinates. By contrast, some
machine tools have five-axis control to shape complex workpart geometries. These
systems typically include three linear axes plus two rotational axes.
The coordinates for a rotational NC system are illustrated in Figure 38.7(b). These
systems are associated with turning operations on NC lathes. Although the work rotates,
this is not one of the controlled axes in a conventional NC turning system. The cutting
path of the tool relative to the rotating workpiece is defined in the x-z plane, as shown in
our figure.
FIGURE 38.7 Coordinate systems used in numerical control: (a) for flat and prismatic work, and (b) for
rotational work.
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In many NC systems, the relative movements between the processing tool and the
workpart are accomplished by fixing the part to a worktable and then controlling the
positions and motions of the table relative to a stationary or semistationary workhead.
Most machine tools and component insertion machines are based on this method of
operation. In other systems, the workpart is held stationary and the work head is moved
along two or three axes. Flame cutters, x-y plotters, and coordinate measuring machines
operate in this mode.
Motion control systems based on NC can be divided into two types: (1) point-to-
point and (2) continuous path. Point-to-point systems, also called positioning systems,
move the workhead (or workpiece) to a programmed location with no regard for the
path taken to get to that location. Once the move is completed, some processing
action is accomplished by the workhead at the location, such as drilling or punching a
hole. Thus, the program consists of a series of point locations at which operations are
performed.
Continuous path systems provide continuous simultaneous control of more than
one axis, thus controlling the path followed by the tool relative to the part. This permits
the tool to perform a process while the axes are moving, enabling the system to generate
angular surfaces, two-dimensional curves, or three-dimensional contours in the workpart.
This operating scheme is required in drafting machines, certain milling and turning
operations, and flame cutting. In machining, continuous path control also goes by the
name contouring.
An important aspect of continuous path motion is interpolation, which is
concerned with calculating the intermediate points along a path to be followed by
the workhead relative to the part. Two common forms of interpolation are linear and
circular. Linear interpolation is used for straight line paths, in which the part pro-
grammer specifies the coordinates of the beginning point and end point of the straight
line as well as the feed rate to be used. The interpolator then computes the travel speeds
of the two or three axes that will accomplish the specified trajectory. Circular
interpolation allows the workhead to follow a circular arc by specifying the coordinates
of its beginning and end points together with either the center or radius of the arc. The
interpolator computes a series of small straight line segments that will approximate the
arc within a defined tolerance.
Another aspect of motion control is concerned with whether the positions in the
coordinate system are defined absolutely or incrementally. In absolute positioning,
the workhead locations are always defined with respect to the origin of the axis system. In
incremental positioning, the next workhead position is defined relative to the present
location. The difference is illustrated in Figure 38.8.
FIGURE 38.8 Absolute
vs. incremental position-
ing. The workhead is at
point (2,3) and is to be
moved to point (6,8). In
absolute positioning, the
move is specified by x ¼6,
y ¼8; while inincremental
positioning, the move is
specified by x ¼ 4, y ¼ 5.
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38.3.2 ANALYSIS OF NC POSITIONING SYSTEMS
The function of the positioning system is to convert the coordinates specified in the NC
part program into relative positions between the tool and workpart during processing.
Let us consider how a simple positioning system, shown in Figure 38.9, might operate.
The system consists of a worktable on which a workpart is fixtured. The purpose of the
table is to move the part relative to a tool or workhead. To accomplish this purpose, the
worktable is moved linearly by means of a rotating leadscrew that is driven by a motor.
For simplicity, only one axis is shown in our sketch. To provide x-y capability, the
system shown would be piggybacked on top of a second axis perpendicular to the first.
The leadscrew has a certain pitch p, mm/thread (in/thread) or mm/rev (in/rev). Thus,
the table is moved a distance equal to the leadscrew pitch for each revolution. The
velocity at which the worktable moves is determined by the rotational speed of the
leadscrew.
Two basic types of motion control are used in NC: (a) open loop and (b) closed
loop, as shown in Figure 38.10. The difference is that an open-loop system operates
FIGURE 38.9 Motorand
leadscrewarrangement in
an NC positioning system.
FIGURE 38.10 Two
types of motion control in
numerical control: (a)
open loop and (b) closed
loop.
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without verifying that the desired position of the worktable has been achieved. A closed-
loop control system uses feedback measurement to verify that the position of the
worktable is indeed the location specified in the program. Open-loop systems are less
expensive than closed-loop systems and are appropriate when the force resisting the
actuating motion is minimal, as in point-to-point drilling, for example. Closed-loop
systems are normally specified for machine tools that perform continuous path opera-
tions such as milling or turning, in which the resisting forces can be significant.
Open-Loop Positioning Systems To turn the leadscrew, an open-loop positioning
system typically uses a stepping motor (a.k.a. stepper motor). In NC, the stepping motor
is driven by a series of electrical pulses generated by the machine control unit. Each pulse
causes the motor to rotate a fraction of one revolution, called the step angle. The
allowable step angles must conform to the relationship
a ¼
360
n
s
ð38:1Þ
where a ¼ step angle, degrees; and n
s
¼ the number of step angles for the motor, which
must be an integer. The angle through which the motor shaft rotates is given by
A
m
¼ an
p
ð38:2Þ
where A
m
¼ angle of motor shaft rotation, degrees; n
p
¼ number of pulses received by the
motor; and a ¼ step angle, here defined as degrees/pulse. Finally, the rotational speed of the
motor shaft is determined by the frequency of pulses sent to the motor:
N
m
¼
60af
p
360
ð38:3Þ
where N
m
¼ speed of motor shaft rotation, rev/min; f
p
¼ frequency of pulses driving
the stepper motor, Hz (pulses/sec), the constant 60 converts pulses/sec to pulses/min;
the constant 360 converts degrees of rotation to full revolutions; and a ¼ step angle
of the motor, as before.
The motor shaft drives the leadscrewthat determines the position and velocity of the
worktable. Theconnectionis oftendesignedusing agear reductiontoincreasetheprecision
of table movement. However, the angle of rotation and rotational speed of the leadscrew
are reduced by this gear ratio. The relationships are as follows:
A
m
¼ r
g
A
ls
ð38:4aÞ
and
N
m
¼ r
g
N
ls
ð38:4bÞ
where A
m
and N
m
are the angle of rotation, degrees, and rotational speed, rev/min, of the
motor, respectively; A
ls
and N
ls
are the angle of rotation, degrees, and rotational speed,
rev/min, of the leadscrew, respectively; and r
g
¼ gear reduction between the motor shaft
and the leadscrew; for example, a gear reduction of 2 means that the motor shaft rotates
through two revolutions for each rotation of the leadscrew.
The linear position of the table in response to the rotation of the leadscrewdepends
on the leadscrew pitch p, and can be determined as follows:
x ¼
pA
ls
360
ð38:5Þ
where x ¼ x-axis position relative to the starting position, mm (in); p ¼ pitch of the
leadscrew, mm/rev (in/rev); and A
ls
/360 ¼ the number of revolutions (and partial
revolutions) of the leadscrew. By combining Eqs. (38.2), (38.4a), and (38.5) and
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rearranging, the number of pulses required to achieve a specified x-position increment in
a point-to-point system can be found:
n
p
¼
360r
g
x
pa
¼
r
g
n
s
A
ls
360
ð38:6Þ
The velocity of the worktable in the direction of the leadscrew axis can be determined as
follows:
v
t
¼ f
r
¼ N
ls
p ð38:7Þ
where v
t
¼ table travel speed, mm/min (in/min); f
r
¼ table feed rate, mm/min (in/min);
N
ls
¼ rotational speed of the leadscrew, rev/min; and p ¼ leadscrew pitch, mm/rev (in/
rev). The rotational speed of the leadscrew depends on the frequency of pulses driving
the stepping motor:
N
ls
¼
60f
p
n
s
r
g
ð38:8Þ
where N
ls
¼ leadscrew rotational speed, rev/min; f
p
¼ pulse train frequency, Hz (pulses/
sec); n
s
¼ steps/rev, or pulses/rev, and r
g
¼ gear reduction between the motor and the
leadscrew. For a two-axis table with continuous path control, the relative velocities of the
axes are coordinated to achieve the desired travel direction. Finally, the required pulse
frequency to drive the table at a specified feed rate can be obtained by combining Eqs.
(38.7) and (38.8) and rearranging to solve for f
p
:
f
p
¼
v
t
n
s
r
g
60 p
¼
f
r
n
s
r
g
60 p
¼
N
ls
n
s
r
g
60
¼
N
m
n
s
60
ð38:9Þ
Example 38.1
Open-Loop
Positioning
A stepping motor has 48 step angles. Its output shaft is coupled to a leadscrew with a 4:1
gear reduction (four turns of the motor shaft for each turn of the leadscrew). The
leadscrew pitch ¼ 5.0 mm. The worktable of a positioning system is driven by the
leadscrew. The table must move a distance of 75.0 mm fromits current position at a travel
speed of 400 mm/min. Determine (a) how many pulses are required to move the table the
specified distance and (b) the motor speed and (c) pulse frequency required to achieve
the desired table speed.
Solution: (a) To move a distance x ¼75 mm, the leadscrewmust rotate through an angle
calculated as follows:
A
ls
¼
360x
p
¼
360 75 ð Þ
5
¼ 5400

With 48 step angles and a gear reduction of 4, the number of pulses to move the table 75
mm is
n
p
¼
4 48 ð Þ 5400 ð Þ
360
¼ 2880 pulses
(b) Equation (38.7) can be used to find the leadscrew speed corresponding to the table
speed of 400 mm/min,
N
ls
¼
v
t
p
¼
400
5:0
¼ 80:0 rev/min
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The motor speed will be 4 times as fast:
N
m
¼ r
g
N
ls
¼ 4 80 ð Þ ¼ 320 rev/min
(c) Finally, the pulse rate is given by Eq. (38.13):
f
p
¼
320 48 ð Þ
60
¼ 256 Hz
n
Closed-Loop Positioning Systems Closed-loop NC systems, Figure 38.10(b), use
servomotors and feedback measurements to ensure that the desired position is achieved.
A common feedback sensor used in NC (and also industrial robots) is the optical rotary
encoder, illustrated in Figure 38.11. It consists of a light source, a photocell, and a disk
containing a series of slots through which the light source can shine to energize the
photocell. The disk is connected to a rotating shaft, which in turn is connected directly to
the leadscrew. As the leadscrew rotates, the slots cause the light source to be seen by the
photocell as a series of flashes, which are converted into an equivalent series of electrical
pulses. By counting the pulses and computing the frequency of the pulse train, the
leadscrewangle and rotational speed can be determined, and thus worktable position and
speed can be calculated using the pitch of the leadscrew.
The equations describing the operation of a closed-loop positioning system are
analogous to those for an open-loop system. In the basic optical encoder, the angle
between slots in the disk must satisfy the following requirement:
a ¼
360
n
s
ð38:10Þ
where a ¼angle between slots, degrees/slot; and n
s
¼the number of slots in the disk, slots/
rev; and 360 ¼ degrees/rev. For a certain angular rotation of the leadscrew, the encoder
generates a number of pulses given by
n
p
¼
A
ls
a
¼
A
ls
n
s
360
ð38:11Þ
where n
p
¼ pulse count; A
ls
¼ angle of rotation of the leadscrew, degrees; and a ¼ angle
between slots in the encoder, degrees/pulse. The pulse count can be used to determine the
linear x-axis position of the worktable by factoring in the leadscrew pitch. Thus,
x ¼
pn
p
n
s
¼
pA
ls
360
ð38:12Þ
FIGURE 38.11 Optical
encoder: (a) apparatus,
and (b) series of pulses
emitted to measure
rotation of disk.
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Similarly, the feed rate at which the worktable moves is obtained from the frequency of
the pulse train:
v
t
¼ f
r
¼
60 pf
p
n
s
ð38:13Þ
where v
t
¼ table travel speed, mm/min (in/min); f
r
¼ feed rate, mm/min (in/min);
p ¼ pitch, mm/rev (in/rev); f
p
¼ frequency of the pulse train, Hz (pulses/sec); n
s
¼
number of slots in the encoder disk, pulses/rev; and 60 converts seconds to minutes.
The speed relationship given by Eq. (38.7) is also valid for a closed-loop positioning
system.
The series of pulses generated by the encoder is compared with the coordinate
position and feed rate specified in the part program, and the difference is used by the
machine control unit to drive a servomotor that in turn drives the leadscrewand worktable.
As with the open-loop system, a gear reduction between the servomotor and the leadscrew
can also be used, so Eqs. (38.4) are applicable. A digital-to-analog converter is used to
convert the digital signals used by the MCU into a continuous analog signal to operate the
drive motor. Closed-loop NC systems of the type described here are appropriate when
there is force resisting the movement of the table. Most metal-machining operations fall
into this category, particularly those involving continuous path control such as milling and
turning.
Example 38.2 NC
Closed-Loop
Positioning
An NC worktable is driven by a closed-loop positioning system consisting of a
servomotor, leadscrew, and optical encoder. The leadscrew has a pitch ¼ 5.0 mm
and is coupled to the motor shaft with a gear ratio of 4:1 (four turns of the motor for
each turn of the leadscrew). The optical encoder generates 100 pulses/rev of the
leadscrew. The table has been programmed to move a distance of 75.0 mm at a feed
rate ¼400 mm/min. Determine (a) how many pulses are received by the control system
to verify that the table has moved exactly 75.0 mm; and (b) the pulse rate and (c) motor
speed that correspond to the specified feed rate.
Solution: (a) Rearranging Eq. (38.12) to find n
p
,
n
p
¼
xn
s
p
¼
75 100 ð Þ
5
¼ 1500 pulses
(b) The pulse rate corresponding to 400 mm/min can be obtained by rearranging
Eq. (38.13):
f
p
¼
f
r
n
s
60 p
¼
400 100 ð Þ
60 5 ð Þ
¼ 133:33 Hz
(c) Leadscrew rotational speed is the table velocity divided by the pitch:
N
ls
¼
f
r
p
¼ 80 rev/min
With a gear ratio r
g
¼ 4.0, the motor speed N ¼ 4 80 ð Þ ¼ 320 rev/min
n
Precision in Positioning Three critical measures of precision in positioning are control
resolution, accuracy, and repeatability. These terms are most easily explained by con-
sidering a single axis of the position system.
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Control resolution refers to the system’s ability to divide the total range of the axis
movement into closely spaced points that can be distinguished by the control unit.
Control resolution is defined as the distance separating two adjacent control points in
the axis movement. Control points are sometimes called addressable points because
they are locations along the axis to which the worktable can be directed to go. It is
desirable for the control resolution to be as small as possible. This depends on
limitations imposed by (1) the electromechanical components of the positioning
system, and/or (2) the number of bits used by the controller to define the axis coordinate
location.
The electromechanical factors that limit resolution include leadscrew pitch, gear
ratiointhedrivesystem, andthestepangleinasteppingmotor (for anopen-loopsystem) or
the angle between slots in an encoder disk (for a closed-loop system). Together, these
factors determine a control resolution, or minimum distance that the worktable can be
moved. For example, the control resolution for an open-loop system driven by a stepper
motor with a gear reduction between the motor shaft and the leadscrew is given by
CR
1
¼
p
n
s
r
g
ð38:14aÞ
where CR
1
¼ control resolution of the electromechanical components, mm (in); p ¼
leadscrew pitch, mm/rev (in/rev); n
s
¼ number of steps/rev; and r
g
¼ gear reduction.
The corresponding expression for a closed-loop positioning system is similar but
does not include the gear reduction because the encoder is connected directly to the
leadscrew. There is no gear reduction. Thus, control resolution for a closed-loop system is
defined as follows:
CR
1
¼
p
n
s
ð38:14bÞ
where n
s
in this case refers to the number of slots in the optical encoder.
Although unusual in modern computer technology, the second possible factor that
could limit control resolution is the number of bits defining the axis coordinate value. For
example, this limitation may be imposed by the bit storage capacity of the controller. If
B ¼ the number of bits in the storage register for the axis, then the number of control
points into which the axis range can be divided ¼2
B
. Assuming that the control points are
separated equally within the range, then
CR
2
¼
L
2
B
À 1
ð38:15Þ
where CR
2
¼ control resolution of the computer control system, mm (in); and L ¼ axis
range, mm (in). The control resolution of the positioning system is the maximum of the
two values; that is,
CR ¼ Max CR
1
; CR
2
f g ð38:16Þ
It is generally desirable for CR
2
CR
1
, meaning that the electromechanical system is the
limiting factor in control resolution.
When a positioning system is directed to move the worktable to a given control
point, the capability of the system to move to that point will be limited by mechanical
errors. These errors are due to a variety of inaccuracies and imperfections in the
mechanical system, such as play between the leadscrew and the worktable, backlash
in the gears, and deflection of machine components. It is convenient to assume that the
errors form a statistical distribution about the control point that is an unbiased normal
distribution with mean ¼ 0. If we further assume that the standard deviation of the
distribution is constant over the range of the axis under consideration, then nearly all of
the mechanical errors (99.73%) are contained within Æ3 standard deviations of the
Section 38.3/Computer Numerical Control 903
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control point. This is pictured in Figure 38.12 for a portion of the axis range, which
includes three control points.
Given these definitions of control resolution and mechanical error distribution, let
us now consider accuracy and repeatability. Accuracy is defined in a worst-case scenario
in which the desired target point lies exactly between two adjacent control points. Since
the system can only move to one or the other of the control points, there will be an error
in the final position of the worktable. If the target were closer to one of the control points,
then the table would be moved to the closer control point and the error would be smaller.
It is appropriate to define accuracy in the worst case. The accuracy of any given axis of a
positioning system is the maximum possible error that can occur between the desired
target point and the actual position taken by the system; in equation form,
Accuracy ¼ 0:5 CR þ 3s ð38:17Þ
where CR ¼ control resolution, mm (in); and s ¼ standard deviation of the error
distribution, mm (in).
Repeatability refers to the capability of a positioning system to return to a given
control point that has been previously programmed. This capability can be measured in
terms of the location errors encountered when the system attempts to position itself at the
control point. Location errors are a manifestation of the mechanical errors of the
positioning system, which are defined by an assumed normal distribution, as described
above. Thus, the repeatability of any given axis of a positioning system can be defined as
the range of mechanical errors associated with the axis; this reduces to
Repeatability ¼ Æ3s ð38:18Þ
Example 38.3
Control
Resolution,
Accuracy, and
Repeatability
Referring back to Example 38.1, the mechanical inaccuracies in the open-loop positioning
system can be described by a normal distribution whose standard deviation ¼ 0.005 mm.
The range of the worktable axis is 550 mm, and there are 16 bits in the binary register used
by the digital controller to store the programmed position. Determine (a) control
resolution, (b) accuracy, and (c) repeatability for the positioning system.
Solution: (a) Control resolution is the greater of CR
1
and CR
2
as defined by Eqs.
(38.14a) and (38.15):
CR
1
¼
p
n
s
r
g
¼
5:0
48 4 ð Þ
¼ 0:0260 mm
CR
2
¼
L
2
B
À 1
¼
550
2
16
À 1
¼
550
65; 535
¼ 0:0084 mm
CR ¼ Max 0:0260; 0:0084 f g ¼ 0:0260 mm
FIGURE 38.12
A portion of a linear
positioning system axis,
with definition of control
resolution, accuracy, and
repeatability.
Control resolution
= CR
Repeatablity = ±3
Axis
CR + 3
1
2
Accuracy =
Control
point
Control
point
Desired target
point
Distribution of
mechanical errors
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(b) Accuracy is given by Eq. (38.17):
Accuracy ¼ 0:5 0:0260 ð Þ þ 3 0:005 ð Þ ¼ 0:0280 mm
(c) Repeatability ¼ Æ 3(0.005) ¼ Æ 0.015 mm.
n
38.3.3 NC PART PROGRAMMING
In machine tool applications, the task of programming the system is called NC part
programming because the program is prepared for a given part. It is usually accom-
plished by someone familiar with the metalworking process who has learned the
programming procedure for the particular equipment in the plant. For other processes,
other terms may be used for programming, but the principles are similar and a trained
individual is needed to prepare the program. Computer systems are used extensively to
prepare NC programs.
Part programming requires the programmer to define the points, lines, and surfaces of
the workpart in the axis system, and to control the movement of the cutting tool relative to
these defined part features. Several part programming techniques are available, the most
important of which are (1) manual part programming, (2) computer-assisted part program-
ming, (3) CAD/CAM-assisted part programming, and (4) manual data input.
Manual Part Programming For simple point-to-point machining jobs, such as drilling
operations, manual programming is often the easiest and most economical method.
Manual part programming uses basic numerical data and special alphanumeric codes to
define the steps in the process. For example, to perform a drilling operation, a command
of the following type is entered:
n010x70:0 y85:5 f175 s500
Each‘‘word’’ inthe statement specifies a detail inthe drilling operation. The n-word(n010)
is simply a sequence number for the statement. The x- and y-words indicate the x and y
coordinate positions (x ¼ 70.0 mm and y ¼ 85.5 mm). The f-word and s-word specify the
feedrateandspindlespeedtobeusedinthe drilling operation(feedrate¼175mm/minand
spindle speed ¼ 500 rev/min). The complete NC part program consists of a sequence of
statements similar to the above command.
Computer-Assisted Part Programming Computer-assisted part programming in-
volves the use of a high-level programming language. It is suited to the programming
of more complex jobs than manual programming. The first part programming language
was APT (Automatically Programmed Tooling), developed as an extension of the
original NC machine tool research and first used in production around 1960.
In APT, the part programming task is divided into two steps: (1) definition of part
geometry and (2) specification of tool path and operation sequence. In step 1, the part
programmer defines the geometry of the workpart by means of basic geometric elements
such as points, lines, planes, circles, and cylinders. These elements are defined using APT
geometry statements, such as
P1 ¼ POINT=25:0; 150:0
L1 ¼ LINE=P1; P2
P1 is a point defined in the x-y plane located at x ¼ 25 mm and y ¼ 150 mm. L1 is a line
that goes through points P1 and P2. Similar statements can be used to define circles,
Section 38.3/Computer Numerical Control 905
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cylinders, and other geometry elements. Most workpart shapes can be described using
statements like these to define their surfaces, corners, edges, and hole locations.
Specification of the tool path is accomplished with APT motion statements.
A typical statement for point-to-point operation is
GOTO=P1
This directs the tool to move from its current location to a position defined by P1,
where P1 has been defined by a previous APT geometry statement. Continuous path
commands use geometry elements such as lines, circles, and planes. For example, the
command
GORGT=L3; PAST; L4
directs the tool to go right (GORGT) along line L3 until it is positioned just past line L4
(of course, L4 must be a line that intersects L3).
Additional APT statements are used to define operating parameters such as feed
rates, spindle speeds, tool sizes, and tolerances. When completed, the part programmer
enters the APT program into the computer, where it is processed to generate low-level
statements (similar to statements prepared in manual part programming) that can be
used by a particular machine tool.
CAD/CAM-Assisted Part Programming The use of CAD/CAM takes computer-
assisted part programming a step further by using a computer graphics system (CAD/
CAM system) to interact with the programmer as the part program is being prepared. In
the conventional use of APT, a complete program is written and then entered into the
computer for processing. Many programming errors are not detected until computer
processing. When a CAD/CAMsystemis used, the programmer receives immediate visual
verification when each statement is entered, to determine whether the statement is
correct. When part geometry is entered by the programmer, the element is graphically
displayed on the monitor. When the tool path is constructed, the programmer can see
exactly how the motion commands will move the tool relative to the part. Errors can be
corrected immediately rather than after the entire program has been written.
Interaction between programmer and programming system is a significant benefit
of CAD/CAM-assisted programming. There are other important benefits of using CAD/
CAM in NC part programming. First, the design of the product and its components may
have been accomplished on a CAD/CAM system. The resulting design database,
including the geometric definition of each part, can be retrieved by the NC programmer
to use as the starting geometry for part programming. This retrieval saves valuable time
compared to reconstructing the part from scratch using the APT geometry statements.
Second, special software routines are available in CAD/CAM-assisted part pro-
gramming to automate portions of the tool path generation, such as profile milling
around the outside periphery of a part, milling a pocket into the surface of a part, surface
contouring, and certain point-to-point operations. These routines are called by the part
programmer as special macro commands. Their use results in significant savings in
programming time and effort.
Manual Data Input Manual data input (MDI) is a method in which a machine operator
enters the part program in the factory. The method involves use of a CRT display with
graphics capability at the machine tool controls. NC part programming statements are
entered using a menu-driven procedure that requires minimum training of the machine
tool operator. Because part programming is simplified and does not require a special staff
of NC part programmers, MDI is a way for small machine shops to economically
implement numerical control into their operations.
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38.3.4 APPLICATIONS OF NUMERICAL CONTROL
Machining is an important application area for numerical control, but the operating
principle of NC can be applied to other operations as well. There are many industrial
processes in which the position of a workhead must be controlled relative to the part or
product being worked on. We divide the applications into two categories: (1) machine
tool applications, and (2) nonmachine tool applications. It should be noted that the
applications are not all identified by the name numerical control in their respective
industries.
In the machine tool category, NC is widely used for machining operations such as
turning, drilling, and milling (Sections 22.2, 22.3, and 22.4, respectively). The use of NC in
these processes has motivated the development of highly automated machine tools called
machining centers, which change their own cutting tools to perform a variety of
machining operations under NC program control (Section 22.5). In addition to machin-
ing, other numerically controlled machine tools include (1) grinding machines (Section
25.1); (2) sheet metal pressworking machines (Section 20.5.2); (3) tube-bending machines
(Section 20.7); and (4) thermal cutting processes (Section 26.3).
In the nonmachine tool category, NC applications include (1) tape-laying machines
and filament-winding machines for composites (Section 15.2.3 and Section 15.4);
(2) welding machines, both arc welding (Section 31.1) and resistance welding (Section
31.2); (3) component-insertion machines in electronics assembly (Sections 35.3 and 35.4);
(4) drafting machines; and (5) coordinate measuring machines for inspection (Section
42.6.1).
Benefits of NC relative to manually operated equipment in these applications
include (1) reduced nonproductive time, which results in shorter cycle times, (2) lower
manufacturing lead times, (3) simpler fixturing, (4) greater manufacturing flexibility, (5)
improved accuracy, and (6) reduced human error.
38.4 INDUSTRIAL ROBOTICS
An industrial robot is a general-purpose programmable machine possessing certain anthro-
pomorphic features. The most obvious anthropomorphic, or human-like, feature is the
robot’s mechanical arm, or manipulator. The control unit for a modern industrial robot is a
computer that can be programmed to execute rather sophisticated subroutines, thus
providing the robot with an intelligence that sometimes seems almost human. The robot’s
manipulator, combined with a high-level controller, allows an industrial robot to perform a
variety of tasks such as loading and unloading production machine, spot welding, and spray
painting. Robots are typically used as substitutes for human workers in these tasks. The first
industrial robot was installed in a die-casting operation at Ford Motor Company. The robot’s
job was to unload die castings from the die-casting machine.
In this section, we consider various aspects of robot technology and applications,
including how industrial robots are programmed to perform their tasks.
38.4.1 ROBOT ANATOMY
An industrial robot consists of a mechanical manipulator and a controller to move it and
perform other related functions. The mechanical manipulator consists of joints and links
that can position and orient the end of the manipulator relative to its base. The controller
unit consists of electronic hardware and software to operate the joints in a coordinated
fashion to execute the programmed work cycle. Robot anatomy is concerned with the
Section 38.4/Industrial Robotics 907